1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus is popular for use in a copier, printer, facsimile, and the like. In the electrophotographic image forming apparatus, a toner image is transferred directly to a recording medium, e.g., paper, or indirectly thereto after a transfer to an intermediate transfer medium. For such a toner image transfer, used is a photoreceptor having a photosensitive layer including a photoconductive substance formed on the surface thereof. The surface of the photoreceptor is subjected to electric charge injection so that it is uniformly charged, and then an electrostatic latent image is so formed as to correspond to image information by going through various image formation processes. The electrostatic latent image is then developed with a supply of developing agent including a toner from a developing section. The result is a toner image for transfer to a recording medium. To fix thus transferred toner image onto the recording medium, a thermal fixing section is generally used to heat and apply pressure to the recording medium. The fixing section includes a heating section, and uses a developer roller or the like.
The thermal-fixing image forming apparatus is exemplified by a type of transferring and fixing a toner image to a recording medium while heating an intermediate transfer belt, and eventually the toner image being a target for transfer onto the intermediate transfer belt. The intermediate transfer belt here is a medium for use for intermediate transfer, and is placed across a heating roller. As an example, refer to Japanese Unexamined Patent Publication JP-A 10-63121 (1998). The image forming apparatus of JP-A10-63121 has characteristics that the power consumption is relatively low. The issue here is that, in the image forming apparatus, a recording medium is not heated, and when a toner image comes into contact with a recording medium at the time of thermal fixing, the toner image is reduced in temperature. This disadvantageously causes not-fully fixing of the toner image, and image degradation such as offset. There is proposed another image forming apparatus of a type of transferring and fixing a toner image to a recording medium while heating both the toner image on an intermediate transfer belt, and the recording medium before the transfer fixing of the toner image. As an example, refer to Japanese Unexamined Patent Publication JP-A2004-151626. The issue here is that, with the image forming apparatus of JP-A 2004-151626, although the toner image is indeed better fixed to the recording medium, the apparatus requires a larger thermal capacity for a heating section as is heating not only the toner image but also the recording medium. This resultantly increases the power consumption, and the resulting increased amount of power consumption makes up more than one half of the entire amount.
However, energy saving is aimed at as a measure for prevention of global warming. With the recent popularity of an electrophotographic image forming apparatus, the reduction of power consumption is also required for the electrophotographic image forming apparatus at the time of toner image fixing to a recording medium. With thermal fixing, because the heating section is used in the apparatus as described above, there needs to increase the heat resistance for other components in the high-temperature apparatus. This resultantly increases the material cost. Also with thermal fixing, no image fixing is possible until a portion for image fixing reaches a predetermined temperature, thereby taking long to derive the temperature of a predetermined value, i.e., taking long warm-up time. Also with thermal fixing, a multicolor toner image takes long to be fixed to a recording medium compared with a single-color toner image. There thus is a demand for shorter fixing time for a multicolor toner image. To meet such a demand, there is proposed wet fixing, which uses a fixing fluid including water and a liquid which can be dissolved or dispersed in water, and can soften or swell a toner. With such wet fixing, a toner image is softened or swollen by the action of a fixing fluid before being attached to a recording medium, and a pressure is applied thereto so that the toner image is fixed to the recording medium. Such wet fixing consumes much less power than thermal fixing, and thus is considered useful in terms of energy conservation. In terms of fixing time for a multicolor toner image, because wet fixing does not require that much amount of heat, the fixing time can be reduced compared with thermal fixing. As such, there are proposed various improvements for such wet fixing.
One proposal is of an image fixing apparatus that applies a fixing fluid only to a portion to be fixed with a toner, and heats the fixing fluid. The fixing fluid is applied from a fixing fluid ejection member formed with a plurality of minute holes, and is directed to a toner image on an intermediate transfer medium or a recording medium. As an example, refer to Japanese Unexamined Patent Publication JP-A 2004-109747. That is, in the image fixing apparatus, a fixing fluid is applied to a toner image on an intermediate transfer medium or a recording medium before a heating process. The concern here is that, at ambient temperature, an unfixed toner image is merely a cluster of toner particles with no physical or chemical bonding. Therefore, when a liquid substance, e.g., fixing fluid, is applied directly to such an unfixed toner image, the toner particles apt to bleed or coagulate before being strongly bound together through softening and/or swelling. This causes the resulting fixed image look blurred at the edges, and have inconsistencies at halftone portions that are supposed to show uniformity, thereby being unable to provide the high image quality. The heating process after the bleeding or coagulation of the toner particles does surely not put the particles back to their original state.
In the image fixing apparatus of JP-A 2004-109747, when an intermediate transfer medium bears thereon a toner image, the intermediate transfer belt, which is an intermediate transfer medium, is subject to water-repellent treatment such as a treatment with fluorine. This allows, when a fixing fluid is applied to an area of the intermediate transfer belt for toner image formation, the fixing fluid to stay at portions where a toner is attached in the toner image formation area, i.e., image sections, but not at portions where no toner is attached in any area between the toner-attached portions, i.e., no-image sections. The problem here is that when a fixing fluid remains only at the image sections in a recording medium as such, the image sections expand and contract but not the no-image sections. There is thus no way of avoiding the image sections and therearound from becoming wrinkled. Especially when a recording medium is paper made by filtering the paper fiber using water, the wrinkles become more pronounced. Although such an inconvenience is surely prevented if with the minimum amount of fixing fluid needed for swelling of the toner, the minimum amount is extraordinarily small, whereby it is difficult to measure the minimum amount with precision. When a fixing fluid is applied only to the image sections, background fogging or the like occur because some of the toner attaches the no-image sections around the image sections, and the toner is not fixed and thus remains on the recording medium, thereby sometimes making a user's hands and clothes dirty.
In the image forming apparatus of JP-A 10-63121, at the time of transferring and fixing a toner image on a transfer belt being heated to a not-heated recording medium, a fixing fluid may be applied to the toner image. The image forming apparatus of JP-A 10-63121, however, requires a large amount of fixing fluid if wanting to increase the adhesion between the toner image and the recording medium, and the bonding among the toner particles. This is because, in the image forming apparatus, no heat supply is made at the time of transferring and fixing a toner image, and no other specific measures are taken. The use of large amount of fixing fluid inevitably causes wrinkles and curling to a recording medium. The use of large amount of fixing fluid also causes the need for frequent supply of a fixing fluid or the need for a large-capacity storage tank for the fixing fluid. This results in poor maintainability or size increase of the apparatus. Considered here is another possible problem if the previous wet-fixing image forming apparatus uses, for fixing of a toner image, a recording medium that is less prone to be permeated with a fixing fluid, e.g., plastic sheet or coated paper including an overhead projector sheet (hereinafter, referred to as “OHP sheet”). In such a case, the adherence of the toner becomes not sufficient, thereby possibly impairing the completion of the resulting toner image.